This invention relates to a process for the production of drawing die bores, preferably in the finishing of hard metal drawing dies with existing crude bores. It further relates to apparatus for carrying out the process of the invention. Such dies are useful in wire drawing, i.e. for the reduction of a rod or wire by pulling it through a round aperture of a die to reduce its diameter while increasing its length.
It is customary in order to facilitate the finishing of drawing die bores to first introduce draw channels into the workpiece with its production. The diameter of these channels is regulated in such a way that for the calibration of the bores and for the achievement of a sufficiently smooth bore surface, the channel diameter exhibits a sufficiently large undersize. This draw channel is coproduced in the production of the initial drawing die as an unfinished piece and is designated as the crude bore.
Many references are known in which devices for the finishing manufacture of drawing dies are described. All of these devices essentially work according to the same principle but have one substantial disadvantage. Either the progress of the drawing die manufacture must be very precisely supervised in order to achieve the desired final size with the required narrow tolerance or, in the introduction of a cylindrical wire together with an abrasive into the drawing die bore for the finishing of the cylindrical bore part, the finishing must take place in small steps in such a way that the widening of the bore is never greater than the corresponding abrasive grain being used.
Thus, for example, German patent specification (DE-PS) No. 445,958 describes a drawing die finishing machine in which an inlet cone and cylindrical bore part of a drawing die may be finished. In this machine, the work tool and workpiece holder are arranged coaxially. The workpiece holder is moved in rotation while the work tool holder performs oscillating movements in axial direction. A substantial disadvantage here is the fact that the conically shaped working surface of the work tool makes definite contact with the surface to be finished only at its lowermost position. Moreover, finishing marks running in circumferential direction on the cone surface are unavoidable. Besides, an exact control of the progress of the work and of the accuracy of bore size is possible only by stopping the machine, something which requires a considerable loss of time.
In order to meet one of the disadvantages noted for the finishing of inlet cones, namely the unfavorable proportion of working time to idle time of the "needle" used as the work tool, it has been variously suggested that the work tool holder be made tiltable about the narrowest point of the finished cone as the turning point (DE-PS No. 527,000; DE-PS No. 560,176; DE-OS No. 2,419,660), so that the conical or even the cylindrical surface of the work tool lays regularly on the cone surface. In this manner, it is also known (DE-PS No. 416 809) to additionally press the work tool by means of a spring force onto the cone surface.
Even with these processes for the production of the inlet cone in drawing die bores, substantial difficulties are unavoidable such as finishing grooves and time-consuming checks on the working progress.
A somewhat different finishing means is described in German patent specification (DE-OS) No. 2,416,717. The work tool holder and the work piece holder rotate here in opposite directions. A calibrating, pointed wire serves as the work tool. For the cone finishing, the work tool axis is tilted. The work tool and work piece remain in uninterrupted contact with each other. After penetration of the drawing die bore, the work tool touchs a contact arranged below the drawing die, whereby the operation is stopped.
Here also a substantial disadvantage occurs in that one cannot achieve an unobjectionable surface free of finishing grooves in circumferential direction. Besides, this process is possible, due to the inclined position of the work tool, only in a production of the inlet cone wherein the diameter of the cylindrically bored portion considerably exceeds that of the work tool.
To be sure, an attempt has been made (DE-PS No. 592,406) to avoid some of the described disadvantages of known devices for the production of the cylindrical bore part. In this case, the drawing die is clamped in a rotating workpiece holder and, coaxially to this, a work tool in the form of a cylindrical calibrating wire pointed at its working end is arranged in a work tool holder and is placed in oscillating movement in axial direction. In order to remove or erode the work surface, the wire used with an abrasive material is continuously moved against the workpiece in longitudinal direction. With this device, it is possible to achieve a larger diameter expansion per work tool, but the adjustment of the work tool movement in eroding material from the bore is so difficult to accomplish that it very frequently results in the twisting off of the very thin wire. It has been common therefore to undertake the final finishing of very fine drawing die bores by hand, something which is very complicated and requires a considerable expenditure for hand labor.